1. Field of the Invention
The present invention relates to a method for manufacturing a variable-throat exhaust turbocharger of an internal combustion engine having a nozzle throat area varying mechanism to vary the blade angle of a plurality of nozzle vanes rotatably supported by a member in a turbine casing and a method for manufacturing constituent members of the nozzle throat area varying mechanism (hereafter referred to as variable-throat mechanism).
2. Description of the Related Art
In variable throat exhaust turbochargers of internal combustion engine for vehicles, it is necessary to reduce wear of sliding surfaces of link members of variable-throat mechanism which operate under high temperature and non-lubricated condition for an extended period, and it is required to adopt wear-resistant material higher in wear resistant property than that of conventionally used material for sliding members or surface-treat the sliding part to increase wear resistance.
Generally, material having high hardness in high temperature such as tungsten base, nickel base, or cobalt base alloy, or ceramics is used as highly wear-resistant material.
On the other hand, the member to which said sliding members are fixed is generally made of stainless steel for ease of plastic forming such as press work. However, said wear-resistant material used for the sliding members is very hard and brittle, so plastic deformation such as caulking can not be applied to fix the sliding members by caulking to the member to which said sliding member is to be fixed as is done conventionally.
Generally, press-in or shrinkage fit is adopted for firm connection of two members. However, temperature of constituent members of the variable-throat mechanism reaches 600° C. or over, so when thermal expansion coefficient of the drive pin is smaller than that of the drive ring, enough interference can not be maintained at the pressed-in part under such a high temperature, and the drive pin slips off. To prevent this, fixing of the drive pins as shown in FIG. 7, 8 has been adopted conventionally. FIGS. 7 represent the connection of a drive ring with drive pins in prior art, (A) is a front view, and (B) is a sectional view in the direction of arrows Z-Z in FIG. 7(A). FIG. 8 is a sectional view showing in detail the part surrounded by the circle V in FIG. 7(B). In FIGS. 7 and FIG. 8, a drive pin 032 made of wear-resistant material is inserted into a through-hole 31 in a drive ring 3 made of steel material such as stainless steel, then the end 032c thereof is welded to be fixed to the drive ring 3. Reference numeral 032 is a portion to be engaged with a lever plate not shown in the drawings.
Further, firm fixing of the drive pin to the drive ring by caulking is disclosed in Japanese Laid-Open Patent Application NO. 9-112511 (hereafter referred to as patent literature 1).
According to the disclosure, a plurality of notch grooves are provided on the inner periphery of a through hole, and an end of a shaft inserted into the through-hole is plastically deformed by caulking so that the deformed parts of the outer periphery intrude into the notched grooves to function as locking claws to prevent rotation of the shaft.
In the field of small exhaust turbochargers used for internal combustion engines for vehicles, an exhaust turbocharger is disclosed in Japanese Laid-Open Patent Application No. 59-78983 (hereafter referred to as patent literature 2), which is composed such that an exhaust turbine rotor made of ceramic material which operates under high temperature and the compressor made of metal which operates under low temperature are connected by brazing welding.
In this exhaust turbocharger, the end portion of the shaft part of the ceramic turbine rotor is tapered, and a tapering hole is provided in the metal compressor wheel to be engaged tightly with said tapering portion of the turbine rotor, whereby depressed portions are provided on the surface of one or both of the tapering shaft portion and tapering hole so that the tapering shaft portion is connected with the tapering hole by the medium of solder which fills the depressions. By this, highly accurate connection between the ceramic material and metal material is realized, and rigidity of connection between the ceramic turbine rotor and metal compressor wheel is increased by mediacy of solder.
In the field of exhaust turbochargers, particularly small ones used for an internal combustion engines for vehicles, a variable-throat exhaust turbocharger equipped with a variable-throat turbine with which flow rate of exhaust gas entering the turbine rotor through the scroll of the turbine casing can be varied in accordance with operating conditions of the engine in order that the exhaust gas flow matches the optimal operation condition of the turbocharger, has spread widely in recent years.
In the variable-throat turbocharger like this, among the constituent members of the variable-throat mechanism to vary the blade angle of a plurality of the nozzle vanes, drive pins to be engaged with lever plates, each of the lever plates being connected to the nozzle vanes by the medium of each of nozzle shafts, are fixed to a drive ring as shown in FIG. 7, 8 or as disclosed in patent literature 1.
With prior art as shown in FIGS. 7 and 8, there occurs a problem that when highly wear-resistant metal material is used for the drive pin 032, the material is very sensitive against crack when welding and yield rate is decreased. Therefore, when welding such a drive pin 032 to a drive ring made of steel material such as austenite type stainless steel, stable quality and reliability of a member consisting of a drive ring with drive pins fixed thereto can not be secured, and manufacturing cost increases due to low yield rate of fixing processing of the drive pins to the drive ring.
The method disclosed in patent literature 1 cannot be applied when the drive pins are made of wear-resistant metal material.
Although there has been proposed in patent literature 2 as mentioned above a method of connecting a ceramic turbine rotor to a compressor wheel made of metal by brazing welding as a method of connecting constituent members of an exhaust turbocharger, however, this connecting method includes problems as described hereunder, so it can not be applied to connecting constituent members of a variable throat-area mechanism.
The method of connecting by brazing welding proposed in patent literature 2 is applied to connecting a compressor wheel to a turbine rotor to which high torque is loaded under high temperature, and depressions are provided on the surface of the tapering portion of the turbine rotor shaft and on the surface of the tapering hole of the compressor wheel to fill solder therein. In case that the solder is filled imperfectly in the depressions, fracture may occur at this part.
Therefore, in this method of connecting the turbine rotor to the compressor wheel, it is necessary that the brazing welding in the range of depressions is uniform and reliable, and it will take a good deal of processing hours to connect the turbine rotor to the compressor wheel by brazing welding.
Further, in the method of connecting the turbine rotor to compressor wheel, it is difficult to confirm infiltration and spreading of the solder to the far side range of the depression toward the bottom of the hole in the compressor wheel. So, it is difficult to detect defects in the connection, and stable quality of the turbocharger cannot be maintained.
Therefore, the method of connecting by mean of brazing welding of patent literature 2 is not applicable to connecting constituent members of a variable-throat mechanism.
Further, there is a proposal to make a core part of the drive pin 032 with steel or sintered material and coat the surface of sliding part with wear-resistant material by plasma spraying or the like, however, in the case of a small part such as a drive pin 032, 90% of wear-resistant material scatters away and fly loss of the material is large.